Method and apparatus for round trip synchronous replication using SCSI reads

ABSTRACT

The SCSI WRITE command is a two round trip protocol, which introduces significant latency in synchronous replication environments. Example embodiments of the present invention overcome these and other deficiencies by using SCSI READ commands from the replication site to the production site instead of using SCSI WRITE commands from the production site to the replication site to decrease latency in a synchronous replication environment. SCSI READ commands are single round trip commands. Thus, the number of round trips required to complete each I/O is reduced from two round trips to one round trip by maintaining at least one SCSI READ command from a SCSI initiator to a SCSI target and then responding to at least one of the at least one SCSI READ command at the SCSI target according to the SCSI READ command.

A portion of the disclosure of this patent document may contain commandformats and other computer language listings, all of which are subjectto copyright protection. The copyright owner has no objection to thefacsimile reproduction by anyone of the patent document or the patentdisclosure, as it appears in the Patent and Trademark Office patent fileor records, but otherwise reserves all copyright rights whatsoever.

TECHNICAL FIELD

This application relates to data protection.

BACKGROUND

Computer data is vital to today's organizations, and a significant partof protection against disasters is focused on data protection. Assolid-state memory has advanced to the point where cost of memory hasbecome a relatively insignificant factor, organizations can afford tooperate with systems that store and process terabytes of data.

Conventional data protection systems include tape backup drives, forstoring organizational production site data on a periodic basis. Suchsystems suffer from several drawbacks. First, they require a systemshutdown during backup, since the data being backed up cannot be usedduring the backup operation. Second, they limit the points in time towhich the production site can recover. For example, if data is backed upon a daily basis, there may be several hours of lost data in the eventof a disaster. Third, the data recovery process itself takes a longtime.

Another conventional data protection system uses data replication, bycreating a copy of the organization's production site data on asecondary backup storage system, and updating the backup with changes.The backup storage system may be situated in the same physical locationas the production storage system, or in a physically remote location.Data replication systems generally operate either at the applicationlevel, at the file system level, or at the data block level.

Current data protection systems try to provide continuous dataprotection, which enable the organization to roll back to any specifiedpoint in time within a recent history. Continuous data protectionsystems aim to satisfy two conflicting objectives, as best as possible;namely, (i) minimize the down time, in which the organization productionsite data is unavailable, during a recovery, and (ii) enable recovery asclose as possible to any specified point in time within a recenthistory.

Continuous data protection typically uses a technology referred to as“journaling,” whereby a log is kept of changes made to the backupstorage. During a recovery, the journal entries serve as successive“undo” information, enabling rollback of the backup storage to previouspoints in time. Journaling was first implemented in database systems,and was later extended to broader data protection.

SUMMARY

Example embodiments of the present invention provide a method, anapparatus, and a computer program product for decreasing latency in asynchronous replication environment. For example, the method comprisesmaintaining at least one SCSI READ command from a SCSI initiator to aSCSI target and then responding to at least one of the at least one SCSIREAD command at the SCSI target according to the SCSI READ command.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of the present invention may be betterunder stood by referring to the following description taken intoconjunction with the accompanying drawings in which:

FIG. 1 is a simplified illustration of a data protection system, inaccordance with an embodiment of the present invention;

FIG. 2 is a simplified illustration of a write transaction for ajournal, in accordance with an embodiment of the present invention;

FIG. 3 is a block diagram illustrating synchronous replication usingopen read commands in accordance with an embodiment of the presentinvention;

FIGS. 4-5 are flow diagrams illustrating methods for performingsynchronous replication using open read commands according to respectiveexample embodiments of the present invention;

FIG. 6 is a flow diagram illustrating an example method for respondingto SCSI READ commands according to an example embodiment of the presentinvention;

FIGS. 7A-7B are flow diagrams illustrating an end-to-end data flow for aSCSI READ command according to the example embodiment of the presentinvention illustrate in FIGS. 3A-3B, respectively;

FIG. 8 is a block diagram of an example embodiment apparatus accordingto the present invention; and

FIG. 9 is an illustration of an example embodiment of the presentinvention as embodied in program code.

DETAILED DESCRIPTION

The following definitions are employed throughout the specification andclaims.

BACKUP SITE—a facility where replicated production site data is stored;the backup site may be located in a remote site or at the same locationas the production site;

CLONE—a clone may be a copy or clone of the image or images, drive ordrives of a first location at a second location;

DELTA MARKING STREAM—may mean the tracking of the delta between theproduction and replication site, which may contain the meta data ofchanged locations, the delta marking stream may be kept persistently onthe journal at the production site of the replication, based on thedelta marking data the DPA knows which locations are different betweenthe production and the replica and transfers them to the replica to makeboth sites identical;

DPA—a computer or a cluster of computers that serve as a data protectionappliance, responsible for data protection services including inter aliadata replication of a storage system, and journaling of I/O requestsissued by a host computer to the storage system;

HOST—at least one computer or networks of computers that runs at leastone data processing application that issues I/O requests to one or morestorage systems; a host is an initiator with a SAN;

HOST DEVICE—an internal interface in a host, to a logical storage unit;

IMAGE—a copy of a logical storage unit at a specific point in time;

INITIATOR—a node in a SAN that issues I/O requests;

JOURNAL—a record of write transactions issued to a storage system; usedto maintain a duplicate storage system, and to rollback the duplicatestorage system to a previous point in time;

LOGICAL UNIT—a logical entity provided by a storage system for accessingdata from the storage system;

LUN—a logical unit number for identifying a logical unit; PHYSICALSTORAGE UNIT—a physical entity, such as a disk or an array of disks, forstoring data in storage locations that can be accessed by address;

PRODUCTION SITE—a facility where one or more host computers run dataprocessing applications that write data to a storage system and readdata from the storage system;

SAN—a storage area network of nodes that send and receive I/O and otherrequests, each node in the network being an initiator or a target, orboth an initiator and a target;

SOURCE SIDE—a transmitter of data within a data replication workflow,during normal operation a production site is the source side; and duringdata recovery a backup site is the source side;

SNAPSHOT—a Snapshot may refer to differential representations of animage, i.e. the snapshot may have pointers to the original volume, andmay point to log volumes for changed locations. Snapshots may becombined into a snapshot array, which may represent different imagesover a time period;

SPLITTER/PROTECTION AGENT—may be an agent running either on a productionhost a switch or a storage array which can intercept IO and split themto a DPA and to the storage array, fail IO redirect IO or do any othermanipulation to the IO;

STORAGE SYSTEM—a SAN entity that provides multiple logical units foraccess by multiple SAN initiators;

TARGET—a node in a SAN that replies to I/O requests; TARGET SIDE—areceiver of data within a data replication workflow; during normaloperation a back site is the target side, and during data recovery aproduction site is the target side; and

WAN—a wide area network that connects local networks and enables them tocommunicate with one another, such as the Internet.

DESCRIPTION OF EMBODIMENTS USING A FIVE STATE JOURNALING PROCESS

FIG. 1 is a simplified illustration of a data protection system 100, inaccordance with an embodiment of the present invention. Shown in FIG. 1are two sites; Site I, which is a production site, on the right, andSite II, which is a backup site, on the left. Under normal operation theproduction site is the source side of system 100, and the backup site isthe target side of the system. The backup site is responsible forreplicating production site data. Additionally, the backup site enablesrollback of Site I data to an earlier pointing time, which may be usedin the event of data corruption of a disaster, or alternatively in orderto view or to access data from an earlier point in time.

During normal operations, the direction of replicate data flow goes fromsource side to target side. It is possible, however, for a user toreverse the direction of replicate data flow, in which case Site Istarts to behave as a target backup site, and Site II starts to behaveas a source production site. Such change of replication direction isreferred to as a “failover”. A failover may be performed in the event ofa disaster at the production site, or for other reasons. In some dataarchitectures, Site I or Site II behaves as a production site for aportion of stored data, and behaves simultaneously as a backup site foranother portion of stored data. In some data architectures, a portion ofstored data is replicated to a backup site, and another portion is not.

The production site and the backup site may be remote from one another,or they may both be situated at a common site, local to one another.Local data protection has the advantage of minimizing data lag betweentarget and source, and remote data protection has the advantage is beingrobust in the event that a disaster occurs at the source side.

The source and target sides communicate via a wide area network (WAN)128, although other types of networks are also adaptable for use withthe present invention.

In accordance with an embodiment of the present invention, each side ofsystem 100 includes three major components coupled via a storage areanetwork (SAN); namely, (i) a storage system, (ii) a host computer, and(iii) a data protection appliance (DPA). Specifically with reference toFIG. 1, the source side SAN includes a source host computer 104, asource storage system 108, and a source DPA 112. Similarly, the targetside SAN includes a target host computer 116, a target storage system120, and a target DPA 124.

Generally, a SAN includes one or more devices, referred to as “nodes”. Anode in a SAN may be an “initiator” or a “target”, or both. An initiatornode is a device that is able to initiate requests to one or more otherdevices; and a target node is a device that is able to reply torequests, such as SCSI commands, sent by an initiator node. A SAN mayalso include network switches, such as fiber channel switches. Thecommunication links between each host computer and its correspondingstorage system may be any appropriate medium suitable for data transfer,such as fiber communication channel links.

In an embodiment of the present invention, the host communicates withits corresponding storage system using small computer system interface(SCSI) commands.

System 100 includes source storage system 108 and target storage system120. Each storage system includes physical storage units for storingdata, such as disks or arrays of disks. Typically, storage systems 108and 120 are target nodes. In order to enable initiators to send requeststo storage system 108, storage system 108 exposes one or more logicalunits (LU) to which commands are issued. Thus, storage systems 108 and120 are SAN entities that provide multiple logical units for access bymultiple SAN initiators.

Logical units are a logical entity provided by a storage system, foraccessing data stored in the storage system. A logical unit isidentified by a unique logical unit number (LUN). In an embodiment ofthe present invention, storage system 108 exposes a logical unit 136,designated as LU A, and storage system 120 exposes a logical unit 156,designated as LU B.

In an embodiment of the present invention, LU B is used for replicatingLU A. As such, LU B is generated as a copy of LU A. In one embodiment,LU B is configured so that its size is identical to the size of LU A.Thus for LU A, storage system 120 serves as a backup for source sidestorage system 108. Alternatively, as mentioned hereinabove, somelogical units of storage system 120 may be used to back up logical unitsof storage system 108, and other logical units of storage system 120 maybe used for other purposes. Moreover, in certain embodiments of thepresent invention, there is symmetric replication whereby some logicalunits of storage system 108 are used for replicating logical units ofstorage system 120, and other logical units of storage system 120 areused for replicating other logical units of storage system 108.

System 100 includes a source side host computer 104 and a target sidehost computer 116. A host computer may be one computer, or a pluralityof computers, or a network of distributed computers, each computer mayinclude inter alia a conventional CPU, volatile and non-volatile memory,a data bus, an I/O interface, a display interface and a networkinterface. Generally a host computer runs at least one data processingapplication, such as a database application and an e-mail server.

Generally, an operating system of a host computer creates a host devicefor each logical unit exposed by a storage system in the host computerSAN. A host device is a logical entity in a host computer, through whicha host computer may access a logical unit. In an embodiment of thepresent invention, host device 104 identifies LU A and generates acorresponding host device 140, designated as Device A, through which itcan access LU A. Similarly, host computer 116 identifies LU B andgenerates a corresponding device 160, designated as Device B.

In an embodiment of the present invention, in the course of continuousoperation, host computer 104 is a SAN initiator that issues I/O requests(write/read operations) through host device 140 to LU A using, forexample, SCSI commands. Such requests are generally transmitted to LU Awith an address that includes a specific device identifier, an offsetwithin the device, and a data size. Offsets are generally aligned to 512byte blocks. The average size of a write operation issued by hostcomputer 104 may be, for example, 10 kilobytes (KB); i.e., 20 blocks.For an I/O rate of 50 megabytes (MB) per second, this corresponds toapproximately 5,000 write transactions per second.

System 100 includes two data protection appliances, a source side DPA112 and a target side DPA 124. A DPA performs various data protectionservices, such as data replication of a storage system, and journalingof I/O requests issued by a host computer to source side storage systemdata. As explained in detail hereinbelow, when acting as a target sideDPA, a DPA may also enable rollback of data to an earlier point in time,and processing of rolled back data at the target site. Each DPA 112 and124 is a computer that includes inter alia one or more conventional CPUsand internal memory.

For additional safety precaution, each DPA is a cluster of suchcomputers. Use of a cluster ensures that if a DPA computer is down, thenthe DPA functionality switches over to another computer. The DPAcomputers within a DPA cluster communicate with one another using atleast one communication link suitable for data transfer via fiberchannel or IP based protocols, or such other transfer protocol. Onecomputer from the DPA cluster serves as the DPA leader. The DPA clusterleader coordinates between the computers in the cluster, and may alsoperform other tasks that require coordination between the computers,such as load balancing.

In the architecture illustrated in FIG. 1, DPA 112 and DPA 124 arestandalone devices integrated within a SAN. Alternatively, each of DPA112 and DPA 124 may be integrated into storage system 108 and storagesystem 120, respectively, or integrated into host computer 104 and hostcomputer 116, respectively. Both DPAs communicate with their respectivehost computers through communication lines such as fiber channels using,for example, SCSI commands.

In accordance with an embodiment of the present invention, DPAs 112 and124 are configured to act as initiators in the SAN; i.e., they can issueI/O requests using, for example, SCSI commands, to access logical unitson their respective storage systems. DPA 112 and DPA 124 are alsoconfigured with the necessary functionality to act as targets; i.e., toreply to I/O requests, such as SCSI commands, issued by other initiatorsin the SAN, including inter alia their respective host computers 104 and116. Being target nodes, DPA 112 and DPA 124 may dynamically expose orremove one or more logical units.

As described hereinabove, Site I and Site II may each behavesimultaneously as a production site and a backup site for differentlogical units. As such, DPA 112 and DPA 124 may each behave as a sourceDPA for some logical units, and as a target DPA for other logical units,at the same time.

In accordance with an embodiment of the present invention, host computer104 and host computer 116 include protection agents 144 and 164,respectively. Protection agents 144 and 164 intercept SCSI commandsissued by their respective host computers, via host devices to logicalunits that are accessible to the host computers. In accordance with anembodiment of the present invention, a data protection agent may act onan intercepted SCSI commands issued to a logical unit, in one of thefollowing ways:

-   -   Send the SCSI commands to its intended logical unit;    -   Redirect the SCSI command to another logical unit;    -   Split the SCSI command by sending it first to the respective        DPA. After the DPA returns an acknowledgement, send the SCSI        command to its intended logical unit;    -   Fail a SCSI command by returning an error return code; and    -   Delay a SCSI command by not returning an acknowledgement to the        respective host computer.

A protection agent may handle different SCSI commands, differently,according to the type of the command. For example, a SCSI commandinquiring about the size of a certain logical unit may be sent directlyto that logical unit, while a SCSI write command may be split and sentfirst to a DPA associated with the agent. A protection agent may alsochange its behavior for handling SCSI commands, for example as a resultof an instruction received from the DPA.

Specifically, the behavior of a protection agent for a certain hostdevice generally corresponds to the behavior of its associated DPA withrespect to the logical unit of the host device. When a DPA behaves as asource site DPA for a certain logical unit, then during normal course ofoperation, the associated protection agent splits I/O requests issued bya host computer to the host device corresponding to that logical unit.Similarly, when a DPA behaves as a target device for a certain logicalunit, then during normal course of operation, the associated protectionagent fails I/O requests issued by host computer to the host devicecorresponding to that logical unit.

Communication between protection agents and their respective DPAs mayuse any protocol suitable for data transfer within a SAN, such as fiberchannel, or SCSI over fiber channel. The communication may be direct, orvia a logical unit exposed by the DPA. In an embodiment of the presentinvention, protection agents communicate with their respective DPAs bysending SCSI commands over fiber channel.

In an embodiment of the present invention, protection agents 144 and 164are drivers located in their respective host computers 104 and 116.Alternatively, a protection agent may also be located in a fiber channelswitch, or in any other device situated in a data path between a hostcomputer and a storage system.

What follows is a detailed description of system behavior under normalproduction mode, and under recovery mode.

In accordance with an embodiment of the present invention, in productionmode DPA 112 acts as a source site DPA for LU A. Thus, protection agent144 is configured to act as a source side protection agent; i.e., as asplitter for host device A. Specifically, protection agent 144replicates SCSI I/O requests. A replicated SCSI I/O request is sent toDPA 112. After receiving an acknowledgement from DPA 124, protectionagent 144 then sends the SCSI I/O request to LU A. Only after receivinga second acknowledgement from storage system 108 will host computer 104initiate another I/O request.

When DPA 112 receives a replicated SCSI write request from dataprotection agent 144, DPA 112 transmits certain I/O informationcharacterizing the write request, packaged as a “write transaction”,over WAN 128 to DPA 124 on the target side, for journaling and forincorporation within target storage system 120.

DPA 112 may send its write transactions to DPA 124 using a variety ofmodes of transmission, including inter alia (i) a synchronous mode, (ii)an asynchronous mode, and (iii) a snapshot mode. In synchronous mode,DPA 112 sends each write transaction to DPA 124, receives back anacknowledgement from DPA 124, and in turns sends an acknowledgement backto protection agent 144. Protection agent 144 waits until receipt ofsuch acknowledgement before sending the SCSI write request to LU A.

In asynchronous mode, DPA 112 sends an acknowledgement to protectionagent 144 upon receipt of each I/O request, before receiving anacknowledgement back from DPA 124.

In snapshot mode, DPA 112 receives several I/O requests and combinesthem into an aggregate “snapshot” of all write activity performed in themultiple I/O requests, and sends the snapshot to DPA 124, for journalingand for incorporation in target storage system 120. In snapshot mode DPA112 also sends an acknowledgement to protection agent 144 upon receiptof each I/O request, before receiving an acknowledgement back from DPA124.

For the sake of clarity, the ensuing discussion assumes that informationis transmitted at write-by-write granularity.

While in production mode, DPA 124 receives replicated data of LU A fromDPA 112, and performs journaling and writing to storage system 120. Whenapplying write operations to storage system 120, DPA 124 acts as aninitiator, and sends SCSI commands to LU B.

During a recovery mode, DPA 124 undoes the write transactions in thejournal, so as to restore storage system 120 to the state it was at, atan earlier time.

As described hereinabove, in accordance with an embodiment of thepresent invention, LU B is used as a backup of LU A. As such, duringnormal production mode, while data written to LU A by host computer 104is replicated from LU A to LU B, host computer 116 should not be sendingI/O requests to LU B. To prevent such I/O requests from being sent,protection agent 164 acts as a target site protection agent for hostDevice B and fails I/O requests sent from host computer 116 to LU Bthrough host Device B.

In accordance with an embodiment of the present invention, targetstorage system 120 exposes a logical unit 176, referred to as a “journalLU”, for maintaining a history of write transactions made to LU B,referred to as a “journal”. Alternatively, journal LU 176 may be stripedover several logical units, or may reside within all of or a portion ofanother logical unit. DPA 124 includes a journal processor 180 formanaging the journal.

Journal processor 180 functions generally to manage the journal entriesof LU B. Specifically, journal processor 180 (i) enters writetransactions received by DPA 124 from DPA 112 into the journal, bywriting them into the journal LU, (ii) applies the journal transactionsto LU B, and (iii) updates the journal entries in the journal LU withundo information and removes already-applied transactions from thejournal. As described below, with reference to FIGS. 2 and 3A-3D,journal entries include four streams, two of which are written whenwrite transaction are entered into the journal, and two of which arewritten when write transaction are applied and removed from the journal.

FIG. 2 is a simplified illustration of a write transaction 200 for ajournal, in accordance with an embodiment of the present invention. Thejournal may be used to provide an adaptor for access to storage 120 atthe state it was in at any specified point in time. Since the journalcontains the “undo” information necessary to rollback storage system120, data that was stored in specific memory locations at the specifiedpoint in time may be obtained by undoing write transactions thatoccurred subsequent to such point in time.

Write transaction 200 generally includes the following fields:

-   -   one or more identifiers;    -   a time stamp, which is the date & time at which the transaction        was received by source side DPA 112;    -   a write size, which is the size of the data block;    -   a location in journal LU 176 where the data is entered;    -   a location in LU B where the data is to be written; and    -   the data itself.

Write transaction 200 is transmitted from source side DPA 112 to targetside DPA 124. As shown in FIG. 2, DPA 124 records the write transaction200 in four streams. A first stream, referred to as a DO stream,includes new data for writing in LU B. A second stream, referred to asan DO METADATA stream, includes metadata for the write transaction, suchas an identifier, a date & time, a write size, a beginning address in LUB for writing the new data in, and a pointer to the offset in the dostream where the corresponding data is located. Similarly, a thirdstream, referred to as an UNDO stream, includes old data that wasoverwritten in LU B; and a fourth stream, referred to as an UNDOMETADATA, include an identifier, a date & time, a write size, abeginning address in LU B where data was to be overwritten, and apointer to the offset in the undo stream where the corresponding olddata is located.

In practice each of the four streams holds a plurality of writetransaction data. As write transactions are received dynamically bytarget DPA 124, they are recorded at the end of the DO stream and theend of the DO METADATA stream, prior to committing the transaction.During transaction application, when the various write transactions areapplied to LU B, prior to writing the new DO data into addresses withinthe storage system, the older data currently located in such addressesis recorded into the UNDO stream.

By recording old data, a journal entry can be used to “undo” a writetransaction. To undo a transaction, old data is read from the UNDOstream in a reverse order, from the most recent data to the oldest data,for writing into addresses within LU B. Prior to writing the UNDO datainto these addresses, the newer data residing in such addresses isrecorded in the DO stream.

The journal LU is partitioned into segments with a pre-defined size,such as 1 MB segments, with each segment identified by a counter. Thecollection of such segments forms a segment pool for the four journalingstreams described hereinabove. Each such stream is structured as anordered list of segments, into which the stream data is written, andincludes two pointers—a beginning pointer that points to the firstsegment in the list and an end pointer that points to the last segmentin the list.

According to a write direction for each stream, write transaction datais appended to the stream either at the end, for a forward direction, orat the beginning, for a backward direction. As each write transaction isreceived by DPA 124, its size is checked to determine if it can fitwithin available segments. If not, then one or more segments are chosenfrom the segment pool and appended to the stream's ordered list ofsegments.

Thereafter the DO data is written into the DO stream, and the pointer tothe appropriate first or last segment is updated. Freeing of segments inthe ordered list is performed by simply changing the beginning or theend pointer. Freed segments are returned to the segment pool for re-use.

A journal may be made of any number of streams including less than ormore than 5 streams. Often, based on the speed of the journaling andwhether the back-up is synchronous or a synchronous a fewer or greaternumber of streams may be used.

Image Access

Herein, some information is provided for conventional continuous dataprotection systems having journaling and a replication splitter whichmay be used in one or more embodiments is provided. A replication mayset refer to an association created between the source volume and thelocal and/or remote target volumes, and a consistency group contains oneor more replication sets. A snapshot may be the difference between oneconsistent image of stored data and the next. The exact time for closingthe snapshot may determined dynamically depending on replicationpolicies and the journal of the consistency group.

In synchronous replication, each write may be a snapshot. When thesnapshot is distributed to a replica, it may be stored in the journalvolume, so that is it possible to revert to previous images by using thestored snapshots. As noted above, a splitter mirrors may write from anapplication server to LUNs being protected by the data protectionappliance. When a write is requested from the application server it maybe split and sent to the appliance using a host splitter/driver(residing in the I/O stack, below any file system and volume manager,and just above any multipath driver (such as EMC POWERPATH), through anintelligent fabric switch, through array-based splitter, such as EMCCLARiiON.

There may be a number of image access modes. Image access may be used torestore production from the disaster recovery site, and to roll back toa previous state of the data. Image access may be also to temporarilyoperate systems from a replicated copy while maintenance work is carriedout on the production site and to fail over to the replica. When imageaccess is enabled, host applications at the copy site may be able toaccess the replica.

In virtual access, the system may create the image selected in aseparate virtual LUN within the data protection appliance. Whileperformance may be constrained by the appliance, access to thepoint-in-time image may be nearly instantaneous. The image may be usedin the same way as logged access (physical), noting that data changesare temporary and stored in the local journal. Generally, this type ofimage access is chosen because the user may not be sure which image, orpoint in time is needed. The user may access several images to conductforensics and determine which replica is required. Note that in knownsystems, one cannot recover the production site from a virtual imagesince the virtual image is temporary. Generally, when analysis on thevirtual image is completed, the choice is made to disable image access.

If it is determined the image should be maintained, then access may bechanged to logged access using ‘roll to image.’ When disable imageaccess is disabled, the virtual LUN and all writes to it may bediscarded.

In an embodiment of virtual access with roll image in background, thesystem first creates the image in a virtual volume managed by the dataprotection appliance to provide rapid access to the image, the same asin virtual access. Simultaneously in background, the system may roll tothe physical image. Once the system has completed this action, thevirtual volume may be discarded, and the physical volume may take itsplace. At this point, the system continues to function as if loggedimage access was initially selected. The switch from virtual to physicalmay be transparent to the servers and applications and the user may notsee any difference in access. Once this occurs, changes may be read fromthe physical volume instead of being performed by the appliance. Ifimage access is disabled, the writes to the volume while image accesswas enabled may be rolled back (undone). Then distribution to storagemay continue from the accessed image forward.

In some embodiments in physical logged access, the system rolls backward(or forward) to the selected snapshot (point in time). There may be adelay while the successive snapshots are applied to the replica image tocreate the selected image. The length of delay may depend on how far theselected snapshot is from the snapshot currently being distributed tostorage. Once the access is enabled, hosts may read data directly fromthe volume and writes may be handled through the DPA. The host may readthe undo data of the write and the appliance may store the undo data ina logged access journal. During logged access the distribution ofsnapshots from the journal to storage may be paused. When image accessis disabled, writes to the volume while image access was enabled(tracked in the logged access journal) may be rolled back (undone). Thendistribution to storage may continue from the accessed snapshot forward.

Disable image access may mean changes to the replica may be discarded orthrown away. It may not matter what type of access was initiated, thatis, logged or another type, or whether the image chosen was the latestor an image back in time. Disable image access effectively says the workdone at the disaster recovery site is no longer needed.

Delta Marking

A delta marker stream may contain the locations that may be differentbetween the latest I/O data which arrived to the remote side (thecurrent remote site) and the latest I/O data which arrived at the localside. In particular, the delta marking stream may include metadata ofthe differences between the source side and the target side. Forexample, every I/O reaching the data protection appliance for the source112 may be written to the delta marking stream and data is freed fromthe delta marking stream when the data safely arrives at both the sourcevolume of replication 108 and the remote journal 180 (e.g. DO stream).Specifically, during an initialization process no data may be freed fromthe delta marking stream; and only when the initialization process iscompleted and I/O data has arrived to both local storage and the remotejournal data, may be I/O data from the delta marking stream freed. Whenthe source and target are not synchronized, data may not be freed fromthe delta marking stream. The initialization process may start bymerging delta marking streams of the target and the source so that thedelta marking stream includes a list of all different locations betweenlocal and remote sites. For example, a delta marking stream at thetarget might have data too if a user has accessed an image at the targetsite.

The initialization process may create one virtual disk out of all theavailable user volumes. The virtual space may be divided into a selectednumber of portions depending upon the amount of data needed to besynchronized. A list of ‘dirty’ blocks may be read from the delta markerstream that is relevant to the area currently being synchronized toenable creation of a dirty location data structure. The system may beginsynchronizing units of data, where a unit of data is a constant amountof dirty data, e.g., a data that needs to be synchronized.

The dirty location data structure may provide a list of dirty locationuntil the amount of dirty location is equal to the unit size or untilthere is no data left. The system may begin a so-called ping pongprocess to synchronize the data. The process may transfer thedifferences between the production and replica site to the replica.

A discussion of mirroring may be found in U.S. Pat. No. 7,346,805,entitled “PROTECTION OF MIRRORED DATA,” issued on Mar. 18, 2008 andassigned to EMC Corporation of Hopkinton, Mass., which is herebyincorporated by reference in its entirety.

A discussion of journaling and some techniques associated withjournaling may be found in U.S. Pat. No. 7,516,287, entitled “METHODSAND APPARATUS FOR OPTIMAL JOURNALING FOR CONTINUOUS DATA REPLICATION,”issued on Apr. 7, 2009 and assigned to EMC Corporation of Hopkinton,Mass., which is hereby incorporated by reference in its entirety.

A discussion of dynamically adding storage for a journal may be found inU.S. Pat. No. 7,840,536, entitled “METHODS AND APPARATUS FOR DYNAMICJOURNAL EXPANSION,” issued on Nov. 23, 2010 and assigned to EMCCorporation of Hopkinton, Mass., which is hereby incorporated byreference in its entirety.

Reducing Round Trips of Synchronous Replication Using Reads

In synchronous replication environments over fibre channel (FC)connections, I/Os are sent from a production site to a replication siteusing, for example, Small Computer System Interface (SCSI) WRITEcommands to replicate data written to the production site storage.Although some synchronous replication environments may use InternetProtocol (IP), FC has a lower latency at least on shorter distances.

The problem with the FC protocol is that the write command is a tworound trip protocol. For example, the SCSI write command has two phases.In a first phase (i.e., round trip), metadata for the I/O is sent fromthe SCSI initiator to the SCSI target and the SCSI target responds witha TRANSFER READY command after allocating space to store the data forthe command. In a second phase (i.e., round trip), the SCSI initiatorsends the data to the SCSI target, which is then acknowledged by theSCSI target. However, for synchronous replication environments in whichthere is a great distance between the production site and thereplication site, latency has a significant effect on performance.Although there are FC accelerators that change the protocol (e.g., theSCSI WRITE command does not wait for the TRANSFER READY command from thetarget but rather sends data immediately), specialized hardware isrequired. Further, such implementations are done at a low level of theprotocol (e.g., hardware accelerated).

Example embodiments of the present invention overcome these and otherdeficiencies by using SCSI READ commands from the replication site tothe production site instead of using SCSI WRITE commands from theproduction site to the replication site to decrease latency in asynchronous replication environment. SCSI READ commands are single roundtrip commands, as compared to the two-round-trip SCSI WRITE commandsdescribed above. Thus, as will be described below, the number of roundtrips required to complete each I/O is reduced from two round trips toone round trip.

FIGS. 3A-3 b are block diagrams illustrating synchronous replicationusing open read commands in accordance with example embodiments of thepresent invention.

As illustrated in FIG. 3A, a replication environment 300 includes aproduction site and a replication site. The production site includes ahost 305 which may issue I/Os via a switch 315 to production sitestorage 325. Likewise, the replication site includes a host 310, aswitch 320 and replication site storage 330.

The description of the block diagrams of FIGS. 3A-3B may be read inconjunction with the flow diagrams of FIGS. 4-6 illustrating exampleembodiment methods for performing synchronous replication using openread commands and for responding to SCSI READ commands according torespective example embodiments of the present invention.

As illustrated in FIGS. 3A and 4, replication site storage 330 maintainsat least one SCSI READ command 350 from a SCSI initiator to a SCSItarget (405). In other words, initially, the replication site storage330 issues a plurality of SCSI READ commands 337 to the production sitestorage 325. For example, the replication site storage 330 may issue 16SCSI READ commands to the production site storage 325, each having asize of 64 kilobytes (KB). As will be described below, as the productionsite storage responds to the open SCSI READ commands 350, thereplication site storage 330 may issue new SCSI READ commands 337 sothat there are always, for example, 16 SCSI READ commands maintained atthe production site storage 325. In other embodiments there may beseveral open I/Os of several different sizes, for instance there may be8 64 KB reads and 8 8 KB reads open. The production site storage 325then responds to the at least one SCSI READ command 350 at the SCSItarget according to the SCSI READ command (415).

As illustrated in FIGS. 3B and 4, in certain embodiments, there may be aDPA 335, 340 and a splitter 365, 375 at each site. Alternatively, theDPA 335, 340 may be part of the storage 325, 330. The production siteDPA 335 may maintain a plurality of open read commands 350 and exposes atarget to the replication site DPA 340 through which the replicationsite DPA 340 may send read commands. The storage 325 exposes one or moreLUs to the host 305 and the host 305 can read and write to the LUs. Thereplication mechanism provided by the DPAs 335, 340 replicates allwrites from the production site to the replica site. The replicationsite DPA 340 may read this data from the LU exposed by the productionsite DPA 335 and write it to the journal 380.

Therefore, the replication site gets the data from the production sitein only one round trip. As will be described below, the production siteDPA 335 may respond to the at least one SCSI READ command 350 byaborting the SCSI READ command if the SCSI READ command times out, byresponding with an amount of data less than that requested by the SCSIREAD command, by responding with an amount of data equal to thatrequested by the SCSI READ command, and by responding with an amount ofdata greater than that requested by the SCSI READ command byencapsulating the data in responses to a plurality of SCSI READcommands.

FIG. 5 is a flow diagram illustrating a method for performingsynchronous replication using open read commands according to an exampleembodiment of the present invention. As illustrated in FIG. 5, thereplication site DPA 340 initiates the SCSI READ command from thereplication site DPA 340 (i.e., SCSI initiator) to the production siteDPA 335 (i.e., SCSI target) (505). The production site DPA 335 thendetermines whether data is available satisfying the SCSI READ command(510). Data becomes available when the host 305 generates an I/O tostorage device 325.

If there is data available satisfying the SCSI READ command (512), aswill be discussed below in greater detail with respect to FIG. 6, theproduction site DPA 335 sends the data to the replication site DPA 340(i.e., SCSI initiator) (515). However, if there is not data availablesatisfying the SCSI READ command (513), the production site DPA 335determines whether the SCSI READ command has timed out (520). If theSCSI READ command has not timed out (522), then the production site DPA335 continues to determine whether data is available satisfying the SCSIREAD command (510).

However, if the SCSI READ command has timed out (523), the productionsite DPA 335 may abort the SCSI READ command (525). In certainembodiments, the production site DPA 335 may start a timer for the SCSIREAD command when it is received. Thus, if the time for the timerexceeds a threshold, the SCSI READ command has timed out and theproduction site storage may abort the SCSI READ command (525). It shouldbe noted that leaving an open SCSI READ command to wait indefinitely torespond with available data from the production site is not optimalbecause it causes resources to be reserved for inordinate amount oftime. Traditional SCSI protocols use an ABORT command to abort othercommands, such as READ. However, ABORT is also a full round-trip commandand the target of the command would have to respond. Instead, in certainembodiments of the present invention, to abort the SCSI READ command,the production site DPA 335 may send a STATUS command without data tothe replication site DPA 340. Thus, the replication site DPA 340 thenmay initiate a new SCSI READ command to the production site DPA 335 tomaintain the number of open SCSI READ commands (505).

FIG. 6 is a flow diagram illustrating an example method for respondingto SCSI READ commands according to an example embodiment of the presentinvention. As illustrated in FIG. 6, the production site DPA 335examines a size of available data for responding to one or more openSCSI READ commands 350 (630).

If the size of available data is smaller than the size of the SCSI READcommand (632), the production site DPA 335 appends (i.e., pads) data tothe available data satisfying the SCSI READ command to equal the amountof data requested by the SCSI READ command (635). In some embodiments ifseveral write requests are available at the DPA, the commands may beencapsulates in an answer to one read command from DPA 340. Theproduction site DPA 335 then responds to the SCSI READ command bysending the data and the appended data 337 to the replication site DPA340 (640). In other embodiments, the production site DPA 335 may send onthe data (i.e., without appending or padding the data to equal theamount of data requested by the SCSI READ command). In such anembodiment, the replication site storage may disregard the underruncondition and not consider the response to the SCSI READ command anerror.

If the size of available data is equal to the size of the SCSI READcommand (633), the production site DPA 335 responds to the SCSI READcommand by sending the data 337′ to the replication site DPA 340 (640).

If the size of available data is larger than the size of the SCSI READcommand (634), the production site DPA 335 encapsulates the data in aplurality of responses to open SCSI READ commands 350 (645). Theproduction site DPA 335 then responds to the SCSI READ commands bysending the data encapsulated in the plurality of responses 337″₁,337″₂, 337″₃ to SCSI READ commands to the replication site DPA 340(640).

It should be understood that data satisfying a SCSI READ command maycomprise one or more I/Os. Further, the data satisfying the SCSI READcommand may include both the I/O metadata and the I/O data.

Further, the size of the SCSI READ commands may be adjusted by theproduction site DPA 335 and the replication site DPA 340 via, forexample, a handshaking mechanism. For example, if a lot of data isavailable at the production site storage 325, the production site DPA335 may indicate to the replication site DPA 340 that the SCSI READcommand size should be increased. Likewise, if there is not much dataavailable, the production site DPA 335 may indicate to the replicationsite DPA 340 that the SCSI READ command size should be decreased,thereby reducing the amount of appended data that is transmitted and,alternatively, reducing the chance that a true underrun is overlooked bythe replication site DPA 340 because it thinks the underrun is actuallyjust a small amount of data sent in response to an open SCSI READcommand.

FIG. 7A is a flow diagram illustrating an end-to-end data flow for usinga SCSI READ command according to an example embodiment of the presentinvention, such as that illustrated in FIG. 3A. A host 305 initiates awrite command to target storage 325 (e.g., one of a plurality of uservolumes (i.e., LUs in the storage 325) (705). The storage 325 respondsto one or more of a plurality of open read commands 350 maintained atthe storage 325 issued by replication site storage 330 with the data ofthe write command and the metadata of the write command (i.e., whichtarget LU, offset, and length) (715A). The metadata may be encapsulatedwith the data of the write command. If more than one write command 350is available, data of several write commands may be encapsulated in ananswer to one open read command 350.

The replication storage 330 then issues a new read command 337 to theproduction storage 325 to take the place of the read command that wasresponded to with the write command data and metadata (720A). In certainembodiments, the new read command 337 may have the same characteristicsas the read command that was responded to, thereby allowing theproduction storage 325 to understand that the previous read commandcompleted successfully). The production site storage 325 may determinethe success or failure of the command (as set out in greater detailbelow) (725A). If the commands was successful (727), the production sitestorage 325 then may send a success status to the host 305 (735A). Incase of failure of the response to the read command data (728) to arriveat the replication site storage 330, the production site storage 325 maysend a failure status to the host 305 (745A).

FIG. 7B is a flow diagram illustrating an end-to-end data flow for usinga SCSI READ command according to an example embodiment of the presentinvention, such as that illustrated in FIG. 3B. A host 305 initiates awrite command to target storage 325 (e.g., one of a plurality of uservolumes (i.e., LUs in the storage 325) (705). A splitter 365 interceptsthe write command and sends it to the DPA 335 (710B). The DPA 335responds to one or more of a plurality of open read command 350maintained at the DPA 335 issued by replication site DPA 340 with thedata of the write command and the metadata of the write command (i.e.,which target LU, offset, and length) (715B). The metadata may beencapsulated with the data of the write command. If more than one writecommand 350 is available, data of several write commands may beencapsulated in an answer to one open read command 350.

DPA 340 then issues a new read command 337 to DPA 335 to take the placeof the read command that was responded to with the write command dataand metadata (720B). In certain embodiments, the new read command 337may have the same characteristics as the read command that was respondedto, thereby allowing the DPA 335 to understand that the previous readcommand completed successfully). The production site DPA 335 maydetermine the success or failure of the command (as set out in greaterdetail below) (725B). If the commands was successful (727B), theproduction site DPA 335 then may send a success status to the splitter365 (730), which may then forward the success status to the host 305(735B). In case of failure of the response to the read command data(728B) to arrive at the replication site DPA 340, the replication siteDPA 340 may notify production site DPA 335 that the read command 350that was responded to failed, or by sending a read command withdifferent parameters to the production site DPA 335.

The production site DPA 335 then may send a failure status to thesplitter 365 (740), which may then forward the failure status to thehost 305 (745B).

The replication site DPA 340 does not know when data (i.e., responses toopen read command 350 encapsulating write commands from the productionsite host 305) may arrive and to which logical unit the data may bewritten. Thus, when sending a read command 337 to the production siteDPA 335, the read command offset may indicate an identifier of the readcommand. For example, if there are 16 open read commands outstanding,each command will be sent to a different offset of the LU thatproduction site DPA 335 exposes (e.g., there can be an I/O to offset 1,an I/O to offset 2, an I/O to offset 3, etc.). In other embodiments theDPA may expose 16 LUs and each command will be sent to a different LU.

Further, some status information of the previous response to an openread command 350 may be sent in the high bits of the offset for thesubsequent new read command 337. For example, if previous open readcommand 1 failed, the next time a read command is issued to open readcommand 1, it will not be issued to offset 1 but rather to anotheroffset (e.g., 16384+1). The production site DPA 335 may interpret thisnew offset and determine that the last writes which were sent thoughopen read 1 failed and will send a fail status to the splitter 365. Incertain embodiments, the status may be encapsulated in more than, we canhave say 8 bits say bits 16 to 24, encapsulate the status that the lastread failed with, since in a SCSI READ 16 commands, the command offsethas 64 bits, the offset may encapsulate both the command ID (say the 32least significant bits) and a command status (say the 32 mostsignificant bits), to indicate that command 3 got a failure status wecan set the offset to have 3 in the lowest 32 bits and the status, at 32most significant bits.

The methods and apparatus of this invention may take the form, at leastpartially, of program code (i.e., instructions) embodied in tangiblenon-transitory media, such as floppy diskettes, CD-ROMs, hard drives,random access or read only-memory, or any other machine-readable storagemedium. When the program code is loaded into and executed by a machine,such as the computer of FIG. 8, the machine becomes an apparatus forpracticing the invention. When implemented on one or moregeneral-purpose processors, the program code combines with such aprocessor to provide a unique apparatus that operates analogously tospecific logic circuits. As such a general purpose digital machine canbe transformed into a special purpose digital machine.

FIG. 9 shows Program Logic 955 embodied on a computer-readable medium960 as shown, and wherein the Logic is encoded in computer-executablecode configured for carrying out the reservation service process of thisinvention and thereby forming a Computer Program Product 900.

The logic for carrying out the method may be embodied as part of theaforementioned system, which is useful for carrying out a methoddescribed with reference to embodiments shown in, for example, FIGS.1-7B. For purposes of illustrating the present invention, the inventionis described as embodied in a specific configuration and using speciallogical arrangements, but one skilled in the art will appreciate thatthe device is not limited to the specific configuration but rather onlyby the claims included with this specification.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Accordingly, the present implementations are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

In reading the above description, persons skilled in the art willrealize that there are many apparent variations that can be applied tothe methods and systems described. In the foregoing specification, theinvention has been described with reference to specific exemplaryembodiments thereof. It will, however, be evident that variousmodifications and changes may be made to the specific exemplaryembodiments without departing from the broader spirit and scope of theinvention as set forth in the appended claims. Accordingly, thespecification and drawings are to be regarded in an illustrative ratherthan a restrictive sense.

What is claimed is:
 1. A method comprising: initiating a SCSI READcommand from a SCSI initiator to a SCSI target; canceling the SCSI READcommand from the SCSI initiator to the SCSI target by sending a STATUScommand without data from the SCSI target to the SCSI initiator if atimer for the command exceeds a threshold; initiating a new SCSI READcommand from the SCSI initiator to the SCSI target to maintain the atleast one SCSI READ command; and responding to at least one of the atleast one SCSI READ command at the SCSI target according to the SCSIREAD command.
 2. The method of claim 1 wherein responding to at leastone of the at least one SCSI READ command at the SCSI target accordingto the SCSI READ command comprises: determining whether data satisfyingthe SCSI READ command is available at the SCSI target; and if datasatisfying the SCSI READ command is available at the SCSI target,sending the data to the SCSI initiator.
 3. The method of claim 2 furthercomprising padding the data satisfying the SCSI READ command to equalthe amount of data requested by the SCSI READ command.
 4. The method ofclaim 2 wherein data satisfying the SCSI READ command comprises at leastone I/O.
 5. The method of claim 2 wherein an amount of data available atthe SCSI target is less than an amount of data requested in the SCSIREAD command returning status after a partial part of the data is sentto the initiator, the method further comprising receiving the data fromthe SCSI target without reporting an error.
 6. The method of claim 2wherein a SCSI READ command size is smaller than a SCSI WRITE commandsize, the method further comprising encapsulating the SCSI WRITE commandin a plurality of responses to SCSI READ commands.
 7. The method ofclaim 1 wherein responding to at least one of the at least one SCSI READcommand to the SCSI target according to the SCSI READ command comprisesencapsulating one or more SCSI WRITE commands from the SCSI target tothe SCSI initiator in response to the SCSI READ command, wherein theencapsulated commands include I/O metadata and I/O data.
 8. The methodof claim 1 further comprising: issuing one or more subsequent SCSI READcommands to replace a previous SCSI READ command; encapsulating a statusof the one or more SCSI WRITE commands in the one or more subsequentSCSI READ commands metadata; and encapsulating the SCSI READ command inthe metadata of the read command.
 9. The method of claim 1 wherein datais replicated synchronously, the method further comprising generating aSCSI WRITE command at a host; and wherein responding to at least one ofthe at least one SCSI READ command at the SCSI target according to theSCSI READ command comprises: intercepting the SCSI WRITE command at areplication module at the storage array; and responding to at least oneof the at least one SCSI READ command at the SCSI target according tothe SCSI READ command with data of the SCSI WRITE command.
 10. Themethod of claim 9 wherein maintaining at least one SCSI READ commandfrom a SCSI initiator to a SCSI target comprises receiving new SCSI READcommands to take the place of the at least one of the at least one SCSIREAD command responded to, the method further comprising: receiving astatus of the at least one of SCSI READ command responded to from thenew SCSI READ command metadata; and returning the status of the at leastone of the SCSI WRITE commands to the host.
 11. The method of claim 1wherein data is replicated synchronously, the method further comprisinggenerating a SCSI WRITE command at a host; and wherein responding to atleast one of the at least one SCSI READ command at the SCSI targetaccording to the SCSI READ command comprises: intercepting the SCSIWRITE command at a splitter at the storage array; writing the SCSI WRITEcommand to a data protection appliance; encapsulating the SCSI WRITEcommand at the data protection appliance in a response to at least oneof the at least one SCSI READ command; and returning a status to thehost once status returns in a subsequent read command.
 12. An apparatuscomprising: a processor, and memory encoded with instructions that, whenexecuted by the processor, causes the apparatus to perform theoperations of: initiating a SCSI READ command from a SCSI initiator to aSCSI target; canceling the SCSI READ command from the SCSI initiator tothe SCSI target by sending a STATUS command without data from the SCSItarget to the SCSI initiator if a timer for the command exceeds athreshold; and initiating a new SCSI READ command from the SCSIinitiator to the SCSI target to maintain the at least one SCSI READcommand; and responding to at least one of the at least one SCSI READcommand at the SCSI target according to the SCSI READ command.
 13. Theapparatus of claim 12, wherein responding to at least one of the atleast one SCSI READ command at the SCSI target according to the SCSIREAD command comprises: determining whether data satisfying the SCSIREAD command is available at the SCSI target; and if data satisfying theSCSI READ command is available at the SCSI target, sending the data tothe SCSI initiator.
 14. The apparatus of claim 13, wherein the memory isencoded with instructions that, when executed by the processor, causesthe computer to perform the operations of padding the data satisfyingthe SCSI READ command to equal the amount of data requested by the SCSIREAD command.
 15. The apparatus of claim 13 wherein data satisfying theSCSI READ command comprises at least one I/O.
 16. The apparatus of claim13 wherein an amount of data available at the SCSI target is less thanan amount of data requested in the SCSI READ command, returning statusafter a partial part of the data is sent to the initiator, the methodfurther comprising receiving the data from the SCSI target withoutreporting an error.
 17. The apparatus of claim 13 wherein a SCSI READcommand size is smaller than a SCSI WRITE command size, the methodfurther comprising encapsulating the SCSI WRITE command in a pluralityof responses to SCSI READ commands.
 18. The apparatus of claim 12wherein responding to at least one of the at least one SCSI READ commandto the SCSI target according to the SCSI READ command comprisesencapsulating one or more SCSI WRITE commands from the SCSI target tothe SCSI initiator in response to the SCSI READ command, wherein theencapsulated commands include I/O metadata and I/O data.
 19. Theapparatus of claim 12 wherein the memory is further encoded withinstructions that, when executed by the processor, causes the computerto perform the operations of: issuing one or more subsequent SCSI READcommands to replace a previous SCSI READ command; encapsulating a statusof the one or more SCSI WRITE commands in the one or more subsequentSCSI READ commands metadata; and encapsulating the SCSI READ command inthe metadata of the read command.
 20. The apparatus of claim 12 whereindata is replicated synchronously and wherein the memory is encoded withinstructions that, when executed by the processor, causes the computerto perform the operations of generating a SCSI WRITE command at a host;and wherein responding to at least one of the at least one SCSI READcommand at the SCSI target according to the SCSI READ command comprises:intercepting the SCSI WRITE command at a replication module at thestorage array; and responding to at least one of the at least one SCSIREAD command at the SCSI target according to the SCSI READ command withdata of the SCSI WRITE command.
 21. The apparatus of claim 12 whereindata is replicated synchronously and wherein the memory is encoded withinstructions that, when executed by the processor, cause the computer toperform the operations of generating a SCSI WRITE command at a host; andwherein responding to at least one of the at least one SCSI READ commandat the SCSI target according to the SCSI READ command comprises:intercepting the SCSI WRITE command at a splitter at the storage array;writing the SCSI WRITE command to a data protection appliance;encapsulating the SCSI WRITE command at the data protection appliance ina response to at least one of the at least one SCSI READ command; andreturning a status to the host once status returns in a subsequent readcommand.
 22. The apparatus of claim 21 wherein maintaining at least oneSCSI READ command from a SCSI initiator to a SCSI target comprisesreceiving new SCSI READ commands to take the place of the at least oneof the at least one SCSI READ command responded to and wherein thememory is encoded with instructions that, when executed by theprocessor, causes the computer to perform the operations of: receiving astatus of the at least one of SCSI READ command responded to from thenew SCSI READ command metadata; and returning the status of the at leastone of the SCSI WRITE commands to the host.
 23. A computer programproduct having a non-transitory computer readable storage medium withinstructions encoded thereon that, when executed by a processor of acomputer, causes the computer to perform the operations of: initiating aSCSI READ command from a SCSI initiator to a SCSI target; canceling theSCSI READ command from the SCSI initiator to the SCSI target by sendinga STATUS command without data from the SCSI target to the SCSI initiatorif a timer for the command exceeds a threshold; initiating a new SCSIREAD command from the SCSI initiator to the SCSI target to maintain theat least one SCSI READ command; and responding to at least one of the atleast one SCSI READ command at the SCSI target according to the SCSIREAD command.